1. Field of the Invention
The present invention relates to a thin film transistor and a method for fabricating the same, and more particularly, to a thin film transistor fabricated using nanowires, thereby exhibiting a high mobility and a high reliability, and a method for fabricating the same.
The present invention also relates to a liquid crystal display device including a thin film transistor and a method for manufacturing the same, and more particularly, to a liquid crystal display device including a thin film transistor fabricated using nanowires, thereby exhibiting a high mobility and a high reliability, and a method for manufacturing the same.
2. Discussion of the Related Art
The recent expansion of the information society promoted development of new image display devices capable of eliminating the drawbacks of conventional cathode ray tubes (CRTs), for example, drawbacks caused by a heavy and bulky structure.
Of newly-developed image display devices, flat panel display devices, such as a liquid crystal display (LCD), an organic light emitting diode (OLED), a plasma display panel (PDP), and a surface-conduction electron-emitter display (SED), have been highlighted.
Such a flat panel display device is constituted by several ten thousand to several hundred pixels densely arranged. For a switching device to drive each pixel of the flat panel display device, a thin film transistor (TFT) is widely used.
In the past, for the TFT, an organic TFT having a semiconductor layer made of amorphous silicon was used. However, a nanowire TFT having a semiconductor layer made of nanowires has recently been-highlighted.
As used herein, the term “nanowire” generally refers to a structure made of a conductive or semiconductive material while having at least one cross-section having a height of less than 500 mm, preferably, less than 100 mm, and an aspect ratio (length:width) of 10 or more, preferably, 100 or more (WO 02/17362, WO 02/4801, and WO 01/03208).
Of such nanowires, the semiconductive nanowire may be made of a material selected from the group consisting of Si, Ge, Sn, Se, Te, B, C (including diamond), P, B—C, B—P(BP6), B—Si, Si—C, Si—Ge, Si—Sn and Ge—Sn, SiC, BN/BP/BAs, AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb, BN/BP/BAs, AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb, ZnO/ZnS/ZnSe/ZnTe, CdS/CdSe/CdTe, HgS/HgSe/HgTe, BeS/BeSe/BeTe/MgS/MgSe, GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe, PbTe, CuF, CuCl, CuBr, CuI, AgF, AgCl, AgBr, AgI, BeSiN2, CaCN2, ZnGeP2, CdSnAs2, ZnSnSb2, CuGeP3, CuSi2P3, Si3N4, Ge3N4, Al2O3, Al2CO, and an appropriate combination thereof. Of course, the material of the semiconductive nanowire is not limited to these materials.
Such a nanowire may include a carbon nanotube, or a conductive or semiconductive organic polymer material (for example, pentacene and a transition metal oxide).
When such a nanowire is used for a semiconductor layer, it is possible to implement a TFT having a mobility higher than that of the conventional TFT, which uses amorphous silicon, because the nanowire has a single crystalline structure.
In order to form a TFT using nanowires for a semiconductor layer, in conventional cases, a Langmuir-Blodgett (LB) method using a solution having a surface dispersed with nanowires is mainly used.
In accordance with the Langmuir-Blodgett method, an appropriate surface pressure is applied to the surface of the solution, to form a nanowire-dispersed layer on the surface. A substrate is dipped into the solution, which has the nanowire-dispersed surface. The dipping of the substrate is repeated, to transfer the nanowires to the solid substrate, and thus to form a nanowire layer having a single layer structure or a multilayer structure.
However, when a nanowire semiconductor layer is formed using the Langmuir-Blodgett method, the following problems occur.
That is, the nanowires in the nanowire layer formed on the substrate should be aligned in a certain direction, in order to enable the nanowire layer to function as an effective semiconductor layer. However, it is difficult to align the nanowires in a desired direction, using the Langmuir-Blodgett method.
Also, the nanowire semiconductor layer should be selectively formed only on a channel region. However, it is difficult to selectively form the nanowire semiconductor layer only in the channel region.
FIGS. 7A and 7B illustrate a state in which nanowires are not aligned in a certain direction.
If the nanowire layer is formed in a region other than the channel region, there may be a problem in that the nanowire layer scatters light, or adversely affects the reliability of the device.